This invention relates to the preparation of both positive and negative electrodes for use in secondary electrochemical cells and batteries that can be employed as power sources for electric automobiles and for the storage of electric energy generated during intervals of off-peak power consumption.
Substantial work has been done in the development of secondary, high-capacity electrochemical cells and their electrodes. The cells showing the most promise employ alkali metals or alkaline earth metals, often as alloys, as negative electrode reactants and metal sulfides or other metal chalcogenides as reactants within the positive electrode. For good ionic conductance between electrodes, these cells can be operated at elevated temperatures with molten salt electrolytes.
Examples of high-temperature cells and their various components are disclosed in U.S. Pat. No. 3,887,396 to Walsh et al., entitled "Modular Electrochemical Cell", June 3, 1975; U.S. Pat. No. 3,907,589 to Gay and Martino, entitled "Cathodes for a Secondary Electrochemical Cell", Sept. 23, 1975; and allowed U.S. Pat. No. 3,933,520 Jan. 20, 1976, entitled "Method of Preparing Electrodes with Porous Current Collector Structures and Solid Reactants for Secondary Electrochemical Cells", U.S. Pat. No. 3,947,291 Mar. 30, 1976, to Yao and Walsh, entitled "Electrochemical Cell Assembled in Discharged State". Each of these patents is assigned to the assignee of the present application.
In addition to these high-temperature cells, the present invention is also applicable to other types of cells including those operated at ambient temperatures. Cells that employ particulate electrode reactants, that is electrode active materials, can be provided with the improved structure of the present invention to prevent reactant migration resulting from slumping, drifting or electrodeposition in areas of reduced resistance. For instance, lead-acid batteries with particulate lead or lead oxide as electrode reactant material can beneficially employ the improved electrode structures described herein.
Previous electrodes have been prepared with particulate reactant materials blended with electrolyte and in some instances particles of electrically conductive material to form a paste. In positive electrodes, reactants such as transition metal sulfides, e.g. iron sulfides, cobalt sulfides, nickel sulfides and copper sulfides, can be blended with such as powdered carbon, carbon black or powdered iron. Paste compositions of these types can be pressed or otherwise embedded into electrically conductive mesh or other networks and contained in electrically conductive baskets of materials such as iron or molybdenum.
These electrodes have functioned reasonably well in a horizontal orientation but the electrode materials have tended to sag or settle towards the bottom in vertical alignments. In order to prevent this drift of reactant materials, electrically conductive porous substrates such as foamed nickel or vitreous carbon have been employed as current collector structures and the particulate reactant vibratorily loaded into these structures. Although these electrode structures have provided fair support for reactant materials, some problems have been encountered, even where only slight movement of the reactant has occurred.
For instance, in cells employing lithium, generally in alloy form, as negative electrode reactant and a transition metal sulfide as the positive electrode reactant, even a slight slumping of the reactant material in either the negative or the positive electrode will result in a localized mismatch. The consequence of this mismatch can result in a nonuniform movement of lithium into the negative electrode during the charge cycle. As the lithium concentrates locally, free lithium metal or molten lithium alloy may eventually be formed to electrically short the cell. The drifting or slumping of active material can result not only from gravity but also from electrodeposition during charge, of reactant into portions of the cell that have reduced resistance.
A second electrode mismatch problem occurs where a single positive electrode is disposed between two parallel negative electrodes. If the cell resistance between one negative electrode and the positive electrode is slightly less than the corresponding resistance to the other negative electrode, an excess of lithium will electrodeposit into that negative electrode having reduced resistance. The result will be a concentration of lithium in one negative electrode, with possible molten lithium metal or alloy formation.